This invention relates to acoustic surface wave devices. More particularly, it is concerned with acoustic surface wave devices of the sidestepping type for use as filters.
Acoustic surface wave devices employing piezoelectric materials having suitable properties for propagating acoustic surface waves and having transducers for launching and receiving acoustic surface waves in the material are well-known. Typically, the transducers are arrays of interleaved conductive electrodes deposited on the material. In response to electrical signals an input or transmitting transducer launches acoustic surface waves along a predetermined track on the surface of the material. An output or receiving transducer detects the acoustic surface waves and generates electrical signals in response thereto. Typically acoustic surface wave devices have been employed as delay lines and as filters. Because of the frequency response which can be obtained from a device by suitably designing the configuration of the transducer electrodes, particularly desirable bandpass characteristics can be achieved such as that required of an intermediate frequency filter for use in television receivers.
In the development of acoustic surface wave devices for use as filters various problems have been encountered. For example, in addition to the acoustic surface waves, an input transducer also generates bulk waves in the body of the piezoelectric material. Since bulk waves have a longer transit time through the material before reaching the output transducer, they interfere with the proper operation of the device. This problem is avoided, however, with sidestepping type devices in which the input and output transducers are offset on different tracks or propagation paths. A sidestepping device employs a multistrip coupler which is an array of filamentary conductive elements on the surface of the material between the input and output transducers. The coupler transfers surface wave energy, but not bulk wave energy, from the track of the input transducer to the track of the output transducer.
Another significant problem of acoustic surface wave devices is the presence of "triple transit signals" which result from interaction between the input and output tranducers. In response to the receipt of acoustic energy from the input transducer, the output transducer causes a fraction of the energy to be directed back toward the input transducer. The input transducer re-transmits a portion of this energy to the output transducer. Thus, a greatly reduced but nevertheless noticeable echo signal is received by the output transducer. This signal which transits the distance between the input and output transducer three times distorts the electrical signal produced by the output transducer.
Various techniques have been employed to eliminate or reduce the effects of these triple transit signals. Reflections of acoustic energy from the edges of the electrodes of the transducers can be suppressed by using electrodes having two elements of one-eighth wavelength in width and separation in place of single element electrodes of one-quarter wavelength in width and separation. Other techniques have been devised in attempts to reduce the effects due to the regenerative action of the received energy with the transducers. These techniques include the use of unidirectional transducer structures and the use of methods for increasing insertion loss thereby suppressing the triple transit signals to a much greater extent than the primary signal. However, using these techniques incurs increased costs for larger substrates of piezoelectric material, additional terminals and components, or more complicated fabrication procedures, or cause degradation of the operating characteristics of the device.
One proposed solution for the problem of triple transit signals employing a sidestepping acoustic surface wave device is described in U.S. Pat. No. 3,836,876 to Marshall et al. The disclosed device employs a 3 dB multistrip coupler between the tracks of the offset input and output transducers. A second output transducer identical to the primary output transducer is located adjacent to the primary output transducer along the track from the input transducer at the same distance from the coupler as the primary output transducer. The second output transducer is connected to circuit components which are as equivalent as possible to the output circuitry connected to the principal output transducer.
As is well-understood the 3 dB coupler divides the acoustic energy received from the input transducer so that equal energy is propagated along the two tracks to the two output transducers. The two acoustic surface waves are in quadrature with that directed to the primary output transducer leading. Since the two output transducers and their associated circuitry are identical, they propagate back toward the coupler identical signals which are in quadrature. The coupler divides the energy received on each track into equal signals along the two tracks. The changes in phase cause the acoustic surface waves directed toward the input tranducer to be suppressed, and a resulting surface wave is propagated only along the other track to be absorbed by a mass of absorbing material on that track adjacent to the input transducer. This structure while effective in suppressing triple transit signals is more expensive because of the additional terminals and dummy load components required and the additional assembly costs incurred.